(1) Field of the Invention
The present invention relates to an apparatus for learning and controlling air/fuel ratio in an automobile internal combustion engine having an electronically controlled fuel injection apparatus with an air/fuel ratio feedback control function. More specifically, the present invention relates to an apparatus for controlling and learning the air/fuel ratio, which apparatus can cope with the change in air density caused by change in altitude.
(2) Description of the Related Art
An apparatus for learning and controlling the air/fuel ratio, as disclosed in the specification of U.S. Pat. No. 4,615,319, is incorporated into an automobile having an internal combustion engine having an electronically controlled fuel injection apparatus with an air/fuel ratio feedback control function.
In the control system, a basic fuel injection quantity calculated from a parameter of an engine driving state, which participates in the quantity of air sucked in an engine, is corrected by a feedback correction coefficient set by a proportional-integrating control based on a signal from an air/fuel ratio sensor, such as an O.sub.2 sensor, disposed in the exhaust system of the engine to compute a fuel injection quantity. The air/fuel ratio is feedback-controlled to an aimed air/fuel ratio, according to the above-mentioned conventional technique, and the deviation of the feedback correction coefficient from a reference value during the feedback control of the air/fuel ratio is learned for respective predetermined areas of the engine driving state in order to determine a learning correction coefficient. In computing the fuel injection quantity, the basic fuel injection quantity is corrected by the learning correction coefficient for each area so that the basic air/fuel ratio obtained by the fuel injection quantity computed without correction by the feedback correction coefficient comes to agree with desired air/fuel ratio, and during feedback control of the air/fuel ratio, this is further corrected by the feedback correction coefficient to compute the fuel injection quantity.
According to this conventional technique, during the feedback control of the air/fuel ratio, follow-up delay of the feedback control can be prevented during transient driving, and the desired air/fuel ratio can be precisely obtained when feedback control of the air/fuel ratio stops.
Furthermore, there is known system where the basic fuel injection quantity Tp is determined from the throttle valve opening degree .alpha. and the engine rotation number N, for example, the sucked air flow quantity Q is determined from .alpha. and N by referring to a map and Tp is computed according to the formula of Tp=K.multidot.Q/N(K is a constant). There is also known system where the sucked air flow quantity Q is detected by an air flow meter and the basic fuel injection quantity is computed from the flow quantity Q and the engine rotation number N according to the formula of Tp=K.multidot.Q/N. In a case where a flap type air flow meter (volume flow rate-detecting type) is used as the flow meter, the change in air density is not reflected in computation of the basic fuel injection quantity, but if the above-mentioned learning control is performed, the computation can cope with change in air density due to changes in altitude or temperature of sucked air, insofar as learning is advanced in a good condition.
However, in the case where an automobile abruptly ascends to an upland (mountain) from a low land, since the ascending driving is a kind of transient driving, according to the system where learning is performed for the respective areas of the engine driving state, the area for learning is not fixed and even if learning is possible, learning-possible areas are limited while learning is hardly advanced in the majority of areas. Accordingly, in case of the ordinary driving or re-starting of the engine at a flat ground in the vicinity of the summit of the mountain, because of the control delay in the air/fuel ratio feedback control, an over-rich state in the air/fuel mixture gas is produced. This over-rich state is also produced because of large deviation of the basic air/fuel ratio from the desired air/fuel ratio at the stoppage of the air/fuel ratio feedback control. Occurrence of this over-rich state causes various difficulties such as reduction in drivability, engine stalling and difficulty in starting.
The reason is as follows. Although it is necessary to learn and correct the change of the density of air from the deviation of the feedback correction coefficient from the reference value during the air/fuel ratio feedback control, since the learned deviation includes the deviation of the basic air/fuel ratio which depends on dispersion of parts such as a fuel injecting valve or a throttle body, and since this deviation cannot be separated from deviation due to the change of the air density, the deviation corresponding to the change of the air density, which can be inherently indiscriminately learned, should be learned for respective areas of the driving state of the engine. In a case where the automobile abruptly ascends to an upland, learning for the respective areas is impossible and learning is not substantially advanced.
The premise of learning is that the air/fuel ratio feedback control is carried out. However, in conventional techniques, the air/fuel ratio feedback control is carried out only in the low-rotation low-load region (inclusive of the medium-rotation medium-load region) set as the air/fuel ratio feedback control region. The reason is that if the feedback control to the theoretical air/fuel ratio, that is, the desired air/fuel ratio, is carried out in the high-rotation or high-load region, there is a risk of seizure of the engine or burning of the catalyst by elevation of the temperature. Therefore, in this region, the feedback correction coefficient is clamped and a rich output air/fuel ratio is separately obtained to prevent seizure of the engine.
Accordingly, when the automobile ascends a mountain, the driving is performed mainly in the high-load region and the air/fuel ratio feedback control is hardly performed and, hence, learning is not substantially carried out. This is another reason why the deviation corresponding to the change in air density cannot be promptly learned.